4.9 Spiro Compounds
When two (or more) rings share a single atom, the compound is called a spiro compound, and the shared atom, usually a carbon atom, is called the spiro atom. The name comes from the Latin word for "twist", and refers to the twisted structure of the molecule; because spiro carbon is tetrahedral, the rings are joined at a 90° angle to each other.
Another word for a spiro compound is a spirane.
The nomenclature of spiro compounds is based on how many total atoms the spiro ring system has, with numbers to indicate how many atoms each ring has besides the spiro atom. Here are a few examples:
spiro[4.4]nonane |
spiro[4.5]decane |
spiro[5.5]undecane |
spiro[4.6]undecane |
Remember that these are not flat, but twisted in 3D space. The atoms are numbered starting with one of the atoms connected directly to the spiro atom; if the rings are different sizes, then atom 1 belongs to the smaller ring. Numbers increase around that ring until reaching the spiro atom, then around the other ring, as in the following examples:
spiro[4.4]nonan-1-ol |
spiro[4.4]nonan-2-ol |
3-chloro-spiro[4.4]nonan-1-ol |
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3-chloro-6-amino- spiro[4.4]nonan-1-ol |
3-chloro-6-amino-9-mercapto- spiro[4.4]nonan-1-ol |
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Spiro compounds can be chiral. For one, they can have a chiral center like any other chiral molecule. Second, they can have central chirality like single-ring molecules. But there is also a type of chirality unique to spiro compounds, due to the geometry of the twist. Here are examples of all three types:
(R)-spiro[4.4]nonan-2-ol |
(S)-spiro[4.4]nonan-2-ol |
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cis-spiro[4.4]nonan- 2,3-diol |
(R)-spiro[4.4]nonan- 2,3-diol |
(S)-spiro[4.4]nonan- 2,3-diol |
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(aR)-spiro[4.4]nonan-1,6-dione |
(aS)-spiro[4.4]nonan-1,6-dione |
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